Analysing dam behaviour3 February 2009
Bosko J. Guzina analyses behavioural responses of dams built in tectonically active or potentially active areas
Variation of stress fields and deformations in the zone of the earth’s crust, which may or may not be accompanied by apparent dynamic effects (earthquakes), are created mostly due to tectonic activities. This phenomenon may affect the dam via its foundation through interaction.
As the end of a dam’s operational life approaches, the occurrence of stress field variations and deformations in the earth crust – as well as its cumulative effect on dam behaviour – becomes more probable.
Observation is often necessary to monitor the behaviour of dams and detect possible discrepancies, right from the design stage and throughout the dam’s life. Early notification of changes, identification of their origin, prognosis for further development, and the effect on dam safety is of particular importance.
Determining the actual causes of induced changes in dam behaviour – particularly if observed in the early stage – may prove to be a rather complex multidisciplinary task.
The changes which could occur in the dam may be related to:
• Changes of physical and mechanical properties of dams, their bedrock or both.
• Endogenous processes that took place in the lithosphere and then through bedrock were transferred to dam itself (tectonic activities).
Changes or anomalies in dam behaviour could be similar in both cases mentioned above – and both issues are addressed in this paper.
As anomalies in dam foundation behaviour related to endogenous activities often develop and occur intermittently, they are usually identified only after a period of dam use – when damage has already occurred on a dam or its appurtenant structures.
This paper is intended to help facilitate early discovery and identification of the second phenomenon listed above, and its effects on dam foundation behaviour. In addition, it is intended to encourage the upgrading and modernisation of monitoring systems, and methods of interpreting the data.
Variation of stress fields and deformations in the earth’s crust
The present analysis considers the earth’s crust as a heterogeneous (quasi) half -space divided into matrix blocks by grid discontinuities. Discontinuities are mainly represented by ruptures resulting from tectonic actions and other loadings, or their variations. Each of the two elements of half-space (matrix and discontinuities) are characterized by their specific geo-mechanical properties affecting deformation and stress field of the earth’s crust i.e. their response to the action of exogenous and endogenous processes.
In the earth’s crust, surface stress fields may prove rather uneven and are governed by morphology, temperature changes etc. Variation of deformations in the crust or rocks as a result of the variation of loading are related to the deformation of block matrices and of a system of discontinuities hereinafter called joints. In the latter event, the deformations of discontinuities may occur in two directions – either perpendicular to the plane of discontinuity including aperture, or possible slip along discontinuity. It may even be both. The first deformation is a non-linear function of change of stress perpendicular to the plane joints. In the event of fracture aperture, local breaking of a quasi half space of lithosphere occurs. Discontinuity aperture occurs when, as a result of a change of stress, the stress oriented perpendicular to the discontinuity tends to become tensile.
Stress field in the earth’s upper crust is produced as a result of gravitation, tectonic activities (including diapirism), and temperature changes. Stress field is represented by principal stresses and trajectories of principle maximum stress. Principle stresses in lithosphere, as a rule, increase with depth.
Stress field in tectonically active areas of the earth’s crust observed over time is exposed to changes i.e. it is variable and beyond our control. Location, scope and speed of these changes for a given lithosphere zone and its geo-mechanical characteristics depend on tectonic processes as well as changes of exogenous factors to which it may be exposed (soil erosion, thawing of rock glacier formations, change of hydrostatic pressure, activation of landslides, impounding of man made reservoirs etc).
Variation of stress fields in the earth’s crust is accompanied by corresponding deformations and displacements (including its upper zone) as well as potential changes (local) of its deformation characteristics. The latter changes may occur on a local or regional level, with even or uneven distribution (or both) along discontinuities. Displacements in the earth’s crust may also occur without a significant change of stress field (creeping, slipping etc).
Changes in stress fields occur irregularly with variable intensity, and with or without discernible dynamic effects (earthquakes). In zones crossing trajectories of tectonic driving forces, variation of stress fields and associated deformations may be rather divergent and unpredictable.
Variations in stress field and deformations taking place in the upper zone of the earth’s crust may be modified by dam site morphology. Variation may include residual stress relief with associated deformations and dynamic effects. These changes may occur as a result of endogenous and/or exogenous processes.
In the event that a change of external (exogenous) load in the earth crust coincides with tectonic activities, additional effects associated with their interaction may occur. Impounding and emptying of man-made reservoirs, as well as large fluctuations of ground water tables, may produce differential movement along discontinuities. The latter occurrence is related to a decrease of contact stress.
Stress field changes and earth crust deformations occur with or without differential movements along the existing and newly developed discontinuities. These may be accompanied by dynamic effects. In zones with instable stress field conditions the change of external (exogenous) load may induce additional tectonic effects with accompanying deformation and dynamic effects.
With the exception of large-scale earthquakes, the variation of deformations in the earth’s crust often remains unnoticed. They are observed primarily in lengthy constructions such as roads and tunnels where they are often attributed to soil instability, temperature changes and other influences. Standard geodetic methods, besides levelling, are often not helpful in identifying deformations in the earth’s crust of endogenous origin.
Vertical movements in the superficial zone of the earth’s crust may be related to a change of external loading with or without an induction of tectonic influence on the variation of tectonic loading.
Alterations in piezometric head
Alterations of piezometric head field within the aquifer associated with variations of stress field and deformations in the earth’s crust are the subject of the present study. Here the aquifer denotes a part of the earth’s crust with hydraulically potentially active porosity filled with water i.e., a permeable rock zone saturated with water and with a degree of permeability that allows water to be withdrawn or injected.
Transient changes of piezometric head fields may occur as the result of active tectonic processes, fluctuations of barometric pressure, gravitational forces as well as the influence of other loadings on lithosphere.
For a given change of loading, alterations of piezometric head fields are governed by a type of aquifer as well as the type and characteristics of its hydraulically active porosity.
In this respect we may distinguish the following aquifer types:
• Unconfined or semi confined aquifer whose upper surface is in direct contact with atmospheric pressure.
• Completely confined aquifer with no direct contact with atmospheric pressure.
When a change of tectonic loading occurs, the groundwater in the aquifer takes over a part of the loading. Water retained in joints takes over a part of joints system loading while water held in spongy porosity replaces a part of the rock matrix loading. In unconfined aquifers, the partial take-over of varied tectonic stresses is temporary.
In unconfined (open) aquifers, the change of tectonic load in the aquifer zone results in a temporary change of the piezometric head gradients and migration of water towards free surface, or vice versa, and through hydraulically active porosity. The latter migration of water lasts until the initial changed state of piezometric head fields within the hydraulically active porosity has been established. During the latter process the stress field in the rock matrix and contact stress in the joints system is finally established – all this assuming there were no changes of hydrological conditions (inflow or outflow of water) in the aquifer.
Amplitude and duration of temporary changes in the piezometric head field depends on:
• Magnitude and temporal course of the change of tectonic load (velocity).
• Geometry, hydraulic characteristics and aquifer boundary conditions,
• Deformation characteristics of joint system porosity.
Somewhat accelerated short-term changes of piezometric head field may be expected in the event of earthquake.
Additional (partially) reversible deformations in lithosphere occur as a result of a piezometric head field transient alteration. Pertinent displacements last until a stationary state of piezometric head field is established and final deformation of the lithosphere related to a change of tectonic load of fractured half-space has taken place.
In confined aquifers the change of tectonic loads simultaneously entails a permanent alteration of piezometric head fields, except in the event of a hydraulic break through overlaying impervious layer. The latter may occur in the event of an increase of tectonic load when the aquifer hydrostatic pressure to the overlaying strata exceeds geostatic pressure.
Closed point piezometers with automatic continuous pressure recording are the preferred choice to monitor the potential impact of stress field variations. In certain cases multi-level monitoring (at various borehole depths) is a suitable method for recording piezometric head.
Piezometers should be placed on both sides of the structure to monitor potentially active faults. In limestone aquifers, piezometers should also be placed below the zone of intensive karstification. For confined or semi confined aquifers, open air piezometers or piezometers with as narrow a stand pipe as possible (placed above the aquifer covering layer) could be used to reduce the piezometers inertia.
Use of piezometers as mentioned above should make it possible to identify, and predict, the presence of stress fields variations in the lithosphere in zones with hydraulically effective porosity filled with water. This would also involve continuous automatic recording of piezometric head and with exclusion of hydrologic influences.
Earthquakes are preceded by an alteration of stress fields in the corresponding part of the lithosphere. Depending on the velocity of these changes in the aquifers, they are accompanied by corresponding changes of piezometric head fields. The latter implies an earthquake itself as well as a period which follows the earthquake when the changes which occur are of highest intensity. This means that variation of piezometric fields may either be considered as a prediction and/or a consequence of an earthquake.
When there is an alteration of piezometric heads associated with the change of tectonic load, the contact stresses induced along some discontinuities may, over time, temporarily reduce or follow these changes. The latter may further induce differential displacements along the discontinuities with or without apparent dynamic effects.
Variation of interaction between the dam and the foundation bedrock
The dam foundation bedrock represents the upper surface of the lithosphere and, together with the dam, participates in stress field variations and deformations occurring in the earth crust as a result of tectonic activities .
The term dam in this paper implies all appurtenant facilities including screens, grout curtains, drainage systems, water intakes, discharge or spillway bodies, adjacent hydro power plants etc. The variation of interaction of endogenous origin implies the variation of state of stress and deformations along the dam foundation joint induced by the stress field variations in the zone around the dam.
Following the change of interaction, the dam may not show any noticeable signs of damage for a considerable time.
For a certain change of stress field and deformations in the lithosphere, the change of interaction and subsequent effects on the dam depend on, but are not limited to:
• Morphology of the river valley.
• Characteristics of stress field in the upper part of the lithosphere in the dam zone.
• Geological and geo-mechanical characteristics of the dam foundation.
• Dam structure and layout.
• Original in situ state of stress field in the dam bedrocks.
• Combination of the above factors.
Lithosphere deformations of endogenous origin may develop in the dam zone with or without differential displacements along discontinuities in rock, including their aperture which may or may not be accompanied with creation of new fractures.
Differential displacements along discontinuities may be unfavourable for concrete dams founded on hard rock. In the case of concrete gravity dams constructed in extremely deformable bedrocks, the influence of the interaction could possibly end at foundation bedrock plastic deformations entailing a change in stresses in the dam to certain extent, but without any mechanical damage to the structure.
The variation of interaction may affect the integrity and safety of concrete dams founded partially or fully in hard fractured rock. In this case the dam could represent an obstacle for undisturbed deformation of lithosphere surface and could be exposed to significant loads by means of interaction which it would not be able to bear without deformations and structural damage. The latter is particularly the case for dams constructed in canyons.
The change of interaction may also affect grouting galleries of earthfill dams and grout curtains or screens. This would happen if the change of stress field in the lithosphere results in a widening of the river valley at the dam site followed by the aperture of discontinuities. The latter case could prove rather risky for dams of any type founded in erodible rocks and more so in case of the presence of soluble rocks in dam bedrocks.
Due to a change of interaction of endogenous origin, deformations and displacement along the foundation joint are possible in all directions regardless of deformations associated with hydrostatic pressure against the dam.
A very complex state of displacements and deformations may develop in dams founded in geo-mechanically heterogeneous bedrocks.
In the case of arch dams constructed in steep canyons, differential displacements between the blocks of rocks at the flanks of a dam may not affect the structure for some time.
The effect of stress field variation and deformations in lithosphere of endogenous origin to the change of interaction is usually identified only after a longer period of observation at dams. During a structures lifetime, the changes of interaction may re-occur several times, and not always in the same direction. However any damage or degradation is cumulative.
Early identification of the presence of interaction changes in a dam foundation is not an easy task. More so as such effects often resemble those associated with exogenous influences, including weathering.
Anomalies in foundation bedrock behaviour
Anomalies in dam foundation behaviour may indicate changes of stress field and deformations in the lithosphere of endogenous origin.
In this paper the dam foundation behaviour implies variations of stresses, deformations and displacements, as well as piezometric head field in dam bedrocks and water elevation in the reservoir.
The anomaly of dam foundation behaviour denotes unexpected changes related to:
• Permanent displacements and deformations of dam foundation exceeding the anticipated values.
• Permanent change of the function: displacement and dam foundation deformation versus water elevation in the reservoir i.e. its external loading.
• Unexpected temporary or permanent changes of piezometric head field in the dam bedrock as well as the amount and pattern of percolating water.
When determining the presence of anomalies it is first necessary to determine and eliminate possible errors in measurements and readings as well as data obtained by means of defective or poorly calibrated monitoring equipment.
As a majority of the listed anomalies may be a result of endogenous and exogenous factors, the present paper gives a summary of the anomalies for each factor individually. The aim of the summary is to determine the presence of stress field variation and deformations in a dam of tectonic origin and to recognize their possible influence on the structure and its foundation.
Anomalies of exogenous origin
Water acts on dam bedrocks through hydrostatic pressure. The distribution of load which the dam transfers to the bedrock depends on the type of dam, its structure and layout, geo-mechanical characteristics of the bedrock itself and the initial in situ state of stress in dam bedrocks. Direct hydraulic load on the foundation depends on the established piezometric head field which may be significantly affected by sealing and drainage works in dam foundation bedrocks and their hydraulic effects.
Permanent dam foundation deformations
Permanent deformations on the foundation may also be related to the rock matrix. Over time, permanent deformations of fractured rock under the influence of cyclic loading increase with a progressive decrease of yield (except for the case of dam failure).
In the system of joints when a dam external loading is known, permanent deformation depends on joint morphology, presence of joint filling and its geo-technical characteristics. An increase of permanent deformations may also occur in the event of joint filling material being washed out, resulting in permanent squeezing of fractures.
Permanent alteration of piezometric head fields
Changes in piezometric head fields may be associated with:
• Clogging of joint system.
• Degrading grout curtain and joint system in dam bedrocks.
• Dissolution or erosion of foundation rock matrix.
• Clogging of drainage system (in bedrocks of a dam).
Permanent changes of piezometric fields and seepage of water through the dam bedrocks as a rule occur gradually and may commence at any time during operation of the dam.
Degrading of grout curtains is associated with grout wash out from the joint system or its original filling in the grout curtain zone. In certain cases the resistance of grout mass to wash out may even initially be low or it may get lower over time due to associated physical and chemical processes.
Anomalies as a result of endogenous factors
In this case, possible anomalies depend on stress field variations and deformations in the lithosphere in a wider dam zone, as well as on associated variation of interaction between the dam structure and its bedrock. The general characteristics of anomalies in this case are that they occur generally regardless of the external loading of the dam. Those anomalies occur mainly sporadically and may be initiated during construction of the dam. They are usually identified only in an advanced stage.
Permanent displacement and deformations of dam bedrocks
One of the essential characteristics of those movements and deformations is that changes may occur in all directions regardless of the orientation of the direction of active loading against dam bedrocks. The movements and deformations occur cumulatively and may change both orientation and direction during the lifetime of the dam. Although the dam bedrock deformations generally follow trajectories of the maximum principle stress variation in the lithosphere, the relevant change of interaction in the foundation may prove very heterogeneous.
Deformation changes in earth fill dam bedrocks may be identified through the use of strain gauges placed in the clay core close to foundation as well as through deformations and movements in a grouting gallery, should it exist.
Those deformations may occur as differential displacements of matrix blocks along discontinuities and as apertures of discontinuities governed by the previous state of stress field in dam foundation bedrocks (the latter may be first anticipated in the dam abutment).
Change of deformation characteristics in the foundation
Deformations of fractured (quasi) half-space consist of deformations of rock matrix and those of the fracture system. The relation between strain and stress with joint system is a non-linear function. Subsequently, if a change of stress field and deformations in the dam bedrock due to tectonic action occurs, it would then result in a change in the dam foundation deformation characteristics.
Changes in the dam static foundation model
As a joint system in dam bedrocks cannot withstand tensile stresses when there is hydraulic loading, the dam bedrocks may behave as:
• a) Half-space
• b) Quarter-space
• c) Combination of a) and b) but governed by prevailing in situ state of normal stresses in joints and water elevations in the reservoir.
In the event of case a above, hydraulic loading transferred from the dam, as well as that which directly acts against bedrocks, will be accepted by the wider rock area below and around the dam. In the event of case b, hydraulic loading will accept only the rock below the dam (or clay core at earthfill dams) and downstream from the dam. In the event of case c, with the change of water elevation in the reservoir the static model of foundation may be changed from a via b to c.
In the event of a change of stress field in dam bedrocks resulting from endogenous (tectonic) processes, changes may occur in the static model of the dam.
Permanent changes of piezometric head field and water seepage
Change of stress field in dam bedrocks will result in the change of deformations of the joint system and subsequently the change of hydraulic characteristics of bedrock. The grouting curtain is also subject to deformations. As a consequence, a change of percolation through dam bedrocks and an alteration of piezometric head field entailing a change of hydraulic load of the system of joints in dam bedrock would take place. Opening of joints crossing the curtain may be accompanied by their wash out, furthering increasing percolation through dam bedrocks (the same as in the case of exogenous loading). If a system for control and monitoring percolating water has been provided below the dam, it could prove a more reliable indicator of changes in piezometric head field in dam foundations than individual piezometers.
Permanent damage of the grouting curtain and associated effect of the change of piezometric head field may also occur as a result of propagation of seismic waves.
Transient alterations of piezometric head field
Transient alteration of piezometric head field in dam foundation is a reliable indicator of the change and a sign of variation i.e. change of stress field in the lithosphere in a wider dam zone. Those changes may strongly affect the stability of concrete dams and, in particular, arch dam abutments in the event of an improper or clogged drainage system. The same implies for landslides in and around reservoirs.
A differentiation should be made between transient and short-term changes of stress field associated with seismic waves propagation.
Deformations in dam bedrocks resulting from earthquakes
Seismic waves during earthquakes result in transient changes of stress fields in the earth crust. In the zone around the earthquake epicentre, transient as well as additional permanent deformations of the existing and possibly newly formed discontinuities with permanent additional change of stress field may be induced.
Possible additional permanent changes of interaction in the dam foundation bedrock may also take place. These changes depend on several factors such as river valley morphology, geo-mechanical characteristics of bedrock (matrix and joint system), in situ state of stress field in the bedrock and the type and structure of the dam.
It is obvious that for dams built in tectonically active areas there is a high likelihood of the occurrence of such variations of stress field and deformations in the earth crust that may affect the structure, including its safety and operating life. Anomalies in dam foundation behaviour are probable indicators of a change of stress field and deformations in the lithosphere.
It is therefore essential to determine the presence of anomalies in dam bedrocks, to predict their further development and analyze possible effects on dam integrity. Any rehabilitation measures should of course take into account that identified anomalies could be associated with endogenous (tectonic) or exogenous factors.
Identifying cause and origin of individual identified anomalies may prove a rather complex task as they are similar in both processes. For identifying early changes of stress field and deformation of endogenous origin in dam bedrocks it is important that a proper monitoring system with a high degree of accuracy and reliability is established. The system and pertaining analysis of measurement results is necessary to continuously innovate and upgrade during a dam’s lifetime.
Particular attention should be given to establishing a corresponding system for continuous automatic observation of piezometric head field in dam bedrocks, as the unexpected transient changes in piezometric head field may be regarded as a reliable indicator of a change of stress field in bedrocks of endogenous origin.
When identifying anomalies in dam bedrock behaviour, it is necessary to eliminate the possibility of defective monitoring equipment and ensure accuracy of measurement.
Analysis of anomalies identified in dam foundation behaviour of endogenous origin should include a prognosis of their further development and possible influence on integrity and safety of the dam. This could prove a very difficult task considering a wide spectrum of possible solutions.
Assistance in solving the above problems should be subject of further tectonophysics research, with investigations and studies updated throughout a dam’s life.
Bosko J. Guzina, Civil Engineer, 91, Jove Ilica street, 11000 Belgrade, 38000 Serbia. Tel: +381 11 2493179, Email: [email protected]